1. Field of the Invention
This invention relates to living tissue replacements such as artificial dental crowns, artificial dental roots, artificial bones, bone screws, and artificial air tubes. It also relates to a method for preparing living tissue replacements, a glass material and a molding apparatus for use in the manufacture of living tissue replacements.
2. Prior Art
Various biotic ceramics are used to form living tissue replacements such as artificial dental crowns, artificial dental roots, artificial bones, artificial junctions and bone fillers. Among biotic ceramics, great attention is paid to crystallized glass or glass-ceramics because of good biological affinity and high mechanical strength. As to the crystallized glass for biological use, the following proposals have been made.
Japanese Patent Publication (JP-B) No. 69094/1992 proposes a calcium phosphate system crystallized glass for use as a dental material comprising CaO, P.sub.2 O.sub.5, and Al, with the Ca/P ratio being from 0.35 to 0.49. A shape of glass is formed by centrifugal casting and then heat treated for crystallization. Since this method utilizes centrifugal casting for shaping of glass, the glass must be melted, making it difficult to use a high strength glass composition having a high melting point. Then, the crystallized glass of this proposal is less reliable when applied to living tissue replacements which receive substantial impacts in a repetitive manner. In fact, the calcium phosphate system crystallized glass examples disclosed in the publication are not regarded satisfactory in mechanical strength and a substantial amount of glass component leaches out. The casting process is difficult to ensure dimensional precision since glass experiences considerable shrinkage upon cooling. The casting process also allows bubbles to be introduced into the glass which is then insufficient in strength, often resulting in defective parts due to bubble inclusion. The crystallized glass of this proposal raises problems particularly when applied to artificial dental crowns. While artificial dental roots need not have a shape and size specific to an individual patient so that standard parts can be manufactured on a large scale, artificial dental crowns must be configured to a shape conformal to the deficient site of an individual patient and are thus required to be easily shaped by the dentist or dental technician with simple means. It is, however, difficult for the dentist or dental technician to perform shaping by a casting process. The casting process further has the problem that heat treatment over a long time is necessary since crystallization of glass takes place from an amorphous state after shaping by the casting process. In fact, crystallization took about 10 to 20 hours in the examples disclosed in the publication.
JP-B 36107/1992 discloses crystallized glass for use as artificial bones and dental materials. This crystallized glass has a non-calcium phosphate system composition free of P.sub.2 O.sub.5. It is prepared by molding glass powder, followed by firing and crystallization treatment. With the process of molding and firing glass powder, it is difficult to prepare dental crowns and other parts of complex shape. Since the molding step uses an isostatic press and the firing step uses a high temperature of 1,050.degree. C. as described in the publication, this process is quite difficult to practice in the dental office. The process takes a long time since the firing step uses a slow heating rate of 30 to 60.degree.C./hour and a slow cooling rate of 30 to 120.degree. C./hour. Dimensional precision is low since firing entails a large shrinkage factor. Even when a high strength composition is used, firing of glass powder after molding tends to lower strength. Preparation of glass powder requires cumbersome operation and considerable costs because molten glass must be converted into ribbon shape as by passing through water-cooled rollers. In addition, many voids are left after firing.
Furthermore, wollastonite and diopside are precipitated in the crystallized glass of JP-B 36107/1992 although the amount of diopside precipitated is not specified therein except for only one datum of 40% in Example.
Japanese Patent Application Kokai (JP-A) No. 70244/1987 discloses a dental crown-forming material comprising crystallized glass. It is prepared by casting a molten raw material into a mold and heat treating the molded material. At the end of heat treatment, Na.Mg.sub.3 (Si.sub.3 AlO.sub.10)F.sub.2 grains (mica) having improved mechanical workability and Li.sub.2 O.Al.sub.2 O.sub.3.2SiO.sub.2 grains (.beta.-eucryptite) and Li.sub.2 O.Al.sub.2 O.sub.3.4SiO.sub.2 grains (.beta.-spodumene) having improved mechanical strength precipitate in this dental crown-forming material. TiO.sub.2 and ZrO.sub.2 are added for controlling crystal growth and improving mechanical strength and Fe.sub.2 O.sub.3 and MnO are added to control color. The examples disclosed in this publication achieve a flexural strength of 2,000 to 2,700 kg/cm.sup.2, which is still insufficient. The use of a casting process for molding suffers from problems as mentioned above.
JP-A 12637/1987 discloses a glass ceramic dental crown which is prepared by molding molten glass followed by heat treatment for causing mica and spodumene crystal phases to precipitate out. Although the glass ceramic is alleged to be improved in machinability and mechanical strength, no exemplary evaluation of machinability and mechanical strength is disclosed. The use of a casting process for molding suffers from problems as mentioned above.
JP-A 174340/1991 discloses an artificial dental crown formed of a glass ceramic composition comprising a calcium-potassium mica crystal and at least one of enstatite, akermanite, and diopside crystals, or comprising a calcium-potassium-sodium mica crystal and at least one of enstatite, akermanite, diopside, anorthite, and richterite crystals. This glass ceramic is alleged to have a hardness approximate to natural teeth and improved mechanical strength, mechanical workability, corrosion resistance and light transmittance. The glass ceramic of such a crystal structure has insufficient gloss and raises an aesthetic problem when used as dental crowns. Also bioactivity is insufficient. The publication describes that the step of shaping glass ceramic to a dental crown configuration includes a casting process as well as machining although the casting process raises problems as mentioned above.
JP-A 88744/1991 discloses a glass ceramic composition comprising a barium-calcium mica crystal and at least one of enstatite, forsterite, and diopside crystals or comprising a barium-calcium mica crystal and at least one of enstatite, forsterite, diopside, and tetragonal zirconia crystals. In the examples therein, a maximum flexural strength of 5,000 kg/cm.sup.2 is reported. However, this glass ceramic is intended for machinable ceramic, but not for application to biotic materials such as artificial dental crowns. In fact, the glass ceramic of such a crystal structure has insufficient gloss and raises an aesthetic problem when used as dental crowns.